So it it worth reusing an old crank damper? I have a couple but some have slight degradation to the rubber. Can the rubber be replaced with some new sikaflex or similar?
There is some new ones getting about that are a disaster, they are steel in appearance (not cast) and the 'rubber' appears to be Urethane of some time. They do come apart in use.
I've used second hand ones and quite often, mind you, not on competition stuff though. I know the rubber can crack, as any rubber will and often these are discarded as a 'fail' but they are usually OK. Grab them and have a good feel of the Outer rim relative to the hub. By hand, you won't move a good one at all, flexing or twisting. Using a file, I mark them before fitting, so I make a line across the inner and outer, then they can be reliably checked.
There is a number of firms who recondition them, I haven't checked lately, but Repco used to do them on an exchange basis. For a stock replacement, this is where I'd be going. I wouldn't recommend any home reconditioning methods. Sikaflex or anything like that is way too soft.